Discharge lamp lighting device

ABSTRACT

A discharge lamp driving device capable of detecting a lamp life end reliably in a high or low temperature environment for circuit protection, yet preventing the occurrence of the cataphoresis phenomenon. Impedance elements Z 1  and Z 1  are inserted respectively between one filament ends of individual discharge lamps La 1  and La 2  and a node (the ground) having no high frequency amplitude in order to detect a difference between AC components of individual lamp voltages VLa 1  and VLa 2  in closed loops of the discharge lamps La 1  and La 2  and the impedance elements Z 1  and Z 1  in order to judge whether or not the depletion of the emitter occurs. Thus, it is possible to reliably judge the presence of abnormality even when the amplitudes of the lamp voltages VLa 1  and VLa 2  varies in a range of low to high temperature. Also, since there is no need to provide a DC blocking capacitor to a secondary winding N 2  of a leakage transformer LT 1,  the discharge lamps La 1  and La 2  can be free from the DC component so as to be prevented from causing the cataphoresis phenomenon.

TECHNICAL FIELD

The present invention is directed to a discharge lamp driving devicewith an abnormality detection and protection function of detecting alamp life end for circuit protection.

BACKGROUND ART

(First Prior Art)

FIG. 1 s a circuit diagram showing one example of a prior discharge lampdriving device which is identical in circuit configuration to that shownin FIG. 36 of Japanese Patent Publication No. 8-251942. A rectifier DBcomposed of a diode bridge is connected to an AC power source AC througha surge absorption element ZNR and a filter circuit F. Connected acrossa pulsating output terminals are a high frequency bypassing capacitorC2, a series combination of switching elements Q1 and Q2 in the form offield effect transistors through a series circuit of diodes D5 and D6, aseries combination of a smoothing capacitor C10 and a diode 13, and ahigh frequency bypassing capacitor C11. A series circuit of an inductorL2 and a diode D12 is connected between a connection point of switchingelements Q1 and Q2 and a connection point of smoothing capacitor C10 anddiode D13. A leakage transformer LT1 has a primary winding N1 that isconnected in series with a DC blocking capacitor C3 between the cathodeof diode D5 and the connection point of switching elements Q1 and Q2. Asecondary winding N2 of the leakage transformer LT1 has its one endconnected through a DC blocking capacitor C9 to one of filaments of onedischarge lamp La1, and has its other end connected to one of filamentof the other discharge lamp La2. The other filaments of the twodischarge lamps La1 and La2 are connected at one ends thereof to eachother through an auxiliary winding N3 of the leakage transformer LT1 anda DC blocking capacitor C6. The other ends of the filaments of thedischarge lamps La1 and La2 are connected to each other through aresonant inducing capacitor C7. Further, a harmonic distortion improvingcapacitor C4 is connected across diode D6.

The two switching elements Q1 and Q2 are driven by a control circuit CNTto turn on and off alternately. The leakage transformer LT1 includes anauxiliary winding N4 for detection of lamp voltage of the dischargelamps La1 and La2. The detected voltage induced at the auxiliary windingN4 is rectified by means of a diode D8 and is fed to a detection circuit20 for detection of the lamp voltage. Based upon thus detected lampvoltage, the control circuit CNT varies a switching frequency of theswitching elements Q1 and Q2. In short, the source AC voltage isrectified through rectifier DB of which pulsating output is partiallysmoothed out by a valley-filling power source in the form of a step-downchopper circuit composed of switching element Q2, diode D12, inductorL2, smoothing capacitor C10 and a parasitic diode of switching elementQ1. The partially smoothed DC output is converted into a high frequencyoutput by means of an inverter circuit in the form of a half-bridge typeincluding the switching elements Q1 and Q2. The high frequency output isfed through the leakage transformer LT1 to the discharge lamps La1 andLa2 as a load for driving the same. Further, in this prior art, theharmonic distortion improving capacitor C4 compensates for a voltagedifference between the rectifier DB and the valley-filling power source,while an input voltage is switched on and off by utilization of a highfrequency voltage appearing within the inverter circuit so as to draw inthe input current from the rectifier DB through a resonant circuitcomposed of leakage transformer LT1, capacitor C3, discharge lamps La1and La2, and capacitor C7, and through capacitor C4 for improvingharmonic distortion of the input current. The operation of this priorart is known and therefore not discussed herein.

When the above prior art sees that the discharge lamps La1 or La2reaches to the lamp life end, a protective action is made as follows.That is, when the lamp reaches its lamp life end as a result of thedepletion of the negative thermion radiating material (emitter) coatedon the filaments, the lamp voltage of the discharge lamps La1 and La2increases than in a normal condition. With this result, the voltageinduced at the auxiliary winding N4 of the leakage transformer LT!increases so that the detection circuit 20 gives an abnormalitydetection signal to the control circuit CNT in response to the voltageinduced at the auxiliary winding N4 exceeds a threshold. The controlcircuit CNT responds to the abnormality detection signal for activatingthe inverter circuit to intermittently oscillate, thereby effecting aprotective action of reducing the stress on the circuit.

(Second Prior Art)

FIG. 2 shows a circuit diagram of another prior art which is identicalin configuration to the circuit disclosed in FIG. 15 of a JapanesePatent Publication 2000-100587. The second prior art differs from thefirst prior art in that the inductor L2 forming the step-down choppercircuit is omitted, that diode 12 has its anode connected to aconnection point of smoothing capacitor C10 and diode 13 and has itscathode connected to a connection point of the primary winding N1 of theleakage transformer LT1 and capacitor C3 in order to share the leakagetransformer LT1 with the step-down chopper circuit, and that an outputregulation circuit 21 is added in compensation for a largecharacteristic variation of a driving transformer T2. The outputregulation circuit 21 includes a switching element Qb realized by abipolar transistor connected across a control voltage source E through avariable resistor VR and a collector resistor Re. The switching elementQb has its base connected through a resistor Rd to a point between aresistor Rc and a capacitor Cb which are connected in series between theconnection point of the switching elements Q1, Q2 and the negative poleof the control voltage source E. Connected between the output terminalof the control circuit CNT and the negative pole of the control voltagesource E is a series combination of a diode Da, a resistor Ra, and aswitching element Qa of bipolar transistor. The switching element Qa hasits base connected through a base resistor Rb to a connection point ofcollector resistor Re and variable resistor VR. Further, a capacitor Caand a diode Db are connected in parallel across the series combinationof the switching element Qb and the collector resistor Re, while a diodeDc is connected in a base-emitter path of the switching element Qb.While the one switching element Q2 is off, capacitor Cb is chargedthrough resistor Rc so that switching element Qb is caused to turn on inresponse to the voltage increase across capacitor Cb, thereby turningoff the switching element Qa and giving no influence on the operation ofthe inverter circuit. When the switching element Q2 turns on, theswitching element Qb is turned off so that the control voltage source Eacts to charge capacitor Ca through variable resistor VR. As the voltageacross capacitor Ca increases, the switching element Qa responds to turnon, thereby causing the switching element Q2 to turn off. Accordingly,it is made possible to regulate the on-period of switching element Q2 byvarying the resistance of the variable resistor VR to thereby maintainthe output substantially at a constant level irrespective of the varyingcharacteristic of the driving transformer T2. Also this prior art hasthe same protective action as is made in the first prior art when thelamp life end is reached.

In the second prior art, however, the inclusion of the output regulationcircuit 21 brings about an asymmetry (unbalance) of the on-period of theswitching elements Q1 and Q2 in the normal lamp operating condition,whereby a DC voltage will be applied to capacitor C9 connected in serieswith the discharge lamps La1 and La2. With this result, the DC voltageof the charged capacitor C9 will be superimposed upon the high frequencyoutput of the inverter circuit in the normal lamp operating, leading toa problem of causing a cataphoresis phenomenon particularly at a lowtemperature.

In order to solve the problem, it might be reasonable to removecapacitor C9 connected to the secondary of the leakage transformer LT1.However, this would causes another problem. That is, as the dischargelamp reaches the lamp life end, capacitor C9 accumulates an increasedvoltage so that the lamp voltage of the lamp of negative resistivityincreases to make a great difference in the lamp voltage between thenormal operating condition and the lamp life end condition. Such lampvoltage difference is utilized for detection of the lamp life end.However, in the absence of capacitor C9, the lamp voltage would makeonly a small difference between the normal operating condition and thelamp life end condition, making it difficult to detect the lamp life endparticularly at a high temperature environment.

DISCLOSURE OF THE INVENTION

The present invention has been achieved in view of the above problem andhas an object of providing a discharge lamp driving device which iscapable of detecting the lamp life end reliably at either low or hightemperature environment for circuit protection, yet preventing thecataphoresis phenomenon.

The discharge lamp driving device in accordance with the presentinvention includes a rectifier which rectifies an AC source voltage, asmoothing capacitor which smoothes out a pulsating output of therectifier, an inverter circuit having one or more switching elements forconversion of the smoothed DC output made through the smoothingcapacitor into a high frequency output, and a load circuit including aresonance circuit and a discharge lamp and being supplied with the highfrequency output from the inverter circuit, an output transformer havinga primary connected to an output end of the inverter circuit and havinga secondary connected to one filament end of the discharge lamp, animpedance element inserted between the other filament end of thedischarge lamp and a node having no high frequency amplitude, and anabnormality detection and protection means which detects an amplitude ofthe high frequency output flowing through the discharge lamp and theimpedance element in order to make the circuit protection when thedetected amplitude exceeds a predetermined threshold.

The abnormality detection and protection means judges the lamp life endof the discharge lamp when the amplitude of the high frequency outputflowing through the discharge lamp and the impedance element exceeds thethreshold. Since the impedance element is inserted between the otherfilament end of the discharge lamp and the node having no high frequencyamplitude, reliable detection of the lamp life end can be made for thecircuit protection at either low or high temperature environment.Further, since there is no need to connect a capacitor on the secondaryof the output transformer, the cataphoresis phenomenon can be prevented.

In a preferred embodiment, the impedance element is inserted between theother filament end of the discharge lamp and a positive input terminalof the inverter circuit.

The impedance element may be inserted between the other filament end ofthe discharge lamp and a grounded input terminal or output terminal ofthe inverter circuit.

A plurality of the discharge lamps can be connected in series on thesecondary side of the output transformer.

Each impedance element inserted between the filament of each of theindividual discharge lamp and the node having no high frequencyamplitude is preferred to have substantially the same impedance value.

In case where the plural discharge lamps are connected in series on thesecondary side of the output transformer, the impedance element isinserted between the other filament end of at least one discharge lampand the positive input terminal of the inverter circuit, while anotherimpedance element is inserted between the other filament end of at leastanother discharge lamp and the grounded input terminal or outputterminal of the inverter circuit.

In case where the plural discharge lamps are connected in series on thesecondary side of the output transformer, the abnormality detection andprotection means is set to make the circuit protective action when theamplitude of the high frequency output flowing through anyone of thedischarge lamps and the impedance element exceeds a predeterminedthreshold.

Also in case where the plural discharge lamps are connected in series onthe secondary side of the output transformer, the abnormality detectionand protection means may be configured to detect the amplitude of apotential at a connection point of the filaments of the plural dischargelamps and also detect the amplitude of the high frequency output flowingthrough at least one discharge lamp and the impedance element such thatit can make the circuit protective action when either or both of theamplitudes exceeds a predetermined threshold.

Further, in case where the plural discharge lamps are connected inseries on the secondary side of the output transformer, the abnormalitydetection and protection means is configured to detect the amplitude ofa potential at a connection point of the filaments of the pluraldischarge lamps such that it makes the circuit protective action wheneither of thus detected amplitude or the amplitude of the high frequencyoutput flowing through at least one of the high-voltage and low-voltageside discharge lamps and the impedance element exceeds a predeterminedthreshold.

The impedance element may include a resistor, capacitor, and a seriescombination of a resistor and a capacitor.

When the inverter circuit is of a self-excited type, at least a portionof a driving circuit for driving the inverter circuit can be shared withcomponents of the abnormality detection and protection means, enablingto reduce the number of the circuit components.

Still further, the impedance element can be shared with the resonancecircuit included in the load circuit for reducing the number of thecircuit components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram showing the first prior art;

FIG. 2 is a schematic circuit diagram showing the second prior art;

FIG. 3 is a schematic circuit diagram showing a discharge lamp drivingdevice in accordance with a first embodiment of the present invention;

FIG. 4 is a circuit diagram of a principal portion of the above device;

FIGS. 5A-5F are waveform charts for explaining the circuit operation ata normal condition;

FIGS. 6A-6F are waveform charts for explaining the circuit operation atan emitter depletion condition;

FIG. 7 is a schematic circuit diagram showing a discharge lamp drivingdevice in accordance with a second embodiment of the present invention;

FIG. 8 is a circuit diagram of a principal portion of the above device;

FIG. 9 is a schematic circuit diagram showing a discharge lamp drivingdevice in accordance with a third embodiment of the present invention;

FIG. 10 is a circuit diagram of a principal portion of the above device;

FIG. 11 is a schematic circuit diagram showing a discharge lamp drivingdevice in accordance with a fourth embodiment of the present invention;

FIG. 12 is a circuit diagram of a principal portion of the above device;

FIG. 13 is a schematic circuit diagram showing a discharge lamp drivingdevice in accordance with a fifth embodiment of the present invention;

FIG. 14 is a circuit diagram of a principal portion of the above device;

FIG. 15 is a partly omitted schematic circuit diagram showing adischarge lamp driving device in accordance with a sixth embodiment ofthe present invention;

FIG. 16 is a partly omitted schematic circuit diagram showing adischarge lamp driving device in accordance with a seventh embodiment ofthe present invention;

FIG. 17 is a partly omitted schematic circuit diagram showing adischarge lamp driving device in accordance with an eighth embodiment ofthe present invention;

FIG. 18 is a circuit diagram of a principal portion of the above device;

FIG. 19 is a schematic circuit diagram showing a discharge lamp drivingdevice in accordance with a ninth embodiment of the present invention;

FIG. 20 is a schematic circuit diagram showing a modification of theabove device;

FIG. 21 is a schematic circuit diagram showing another modification ofthe above device;

FIG. 22 is a schematic circuit diagram showing a further modification ofthe above device; and

FIG. 23 is a schematic circuit diagram showing a discharge lamp drivingdevice in accordance with a tenth embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

(First Embodiment)

FIG. 3 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment. A series connectedpair of switching elements Q1 and Q2 and a smoothing capacitor C0 areconnected in parallel across pulsating output terminals of a rectifierDB that is composed of a diode bridge to rectify an AC source voltageAC. A leakage transformer LT1 has a primary winding N1 which isconnected between the high-side output terminal of the rectifier DB anda connection point of the switching elements Q1 and Q2, and has asecondary winding N2 connected to filaments (a) and (d) of dischargelamps La1 and La2 of the same rating. For supplying a pre-heatingcurrent, the leakage transformer LT1 has an auxiliary winding N3 whichis connected through a DC blocking capacitor C3 to the other filaments(b) and (c) of the discharge lamps La1 and La2. A capacitor C2 isconnected to the ends of the filaments (a) and (d) away from the voltagesource such that a resonant load circuit is constituted by the leakagetransformer LT1, capacitor C2 and the discharge lamps La1 and La2.

In the present embodiment, the switching elements Q1 and Q2 iscooperative with the resonant load circuit to realize an invertercircuit INV of a half-bridge type which receives, as an input voltage,the DC voltage smoothed by the smoothing capacitor C0. The half-bridgetype inverter circuit INV is known and is driven by a driver circuit(not shown but including a self-excited type using a drivingtransformer) to turn on and off the switching elements Q1 and Q2alternately at a high frequency, applying a square wave high frequencyvoltage to the resonant load circuit so as to make the use of theresonance by a leakage inductance of the leakage transformer LT1 and theresonant-inducing capacitor C2 of the resonant load circuit to supply ahigh frequency voltage of substantially the sinusoidal waveform foroperating the discharge lamps La1 and La2.

Next, the characterizing features of the present embodiment will beexplained. Impedance elements Z1, Z1 are inserted respectively betweenthe filament (a) of the discharge lamp La1 and a node (ground) having nohigh frequency amplitude and between the filament (d) of the dischargelamp La2 and the node, while an impedance element Z2 is inserted betweenthe filament (b) and the capacitor C3 that is connected between thehigh-side terminal of the rectifier DB and the auxiliary winding N3 ofthe leakage transformer LT1. Further, a series circuit of impedanceelements Z3 and Z4 is connected between the ground and a connectionpoint of the auxiliary winding N3 and the filament (c).

FIG. 4 is a circuit diagram showing the resonant load circuit extractedas a principal portion. The lamp voltages VLa1 and VLa2 supplied to thetwo discharge lamps La1 and La2 are each applied to each closed loopcomposed of the impedance elements Z1, Z3, and Z4. Also, the pulsatingoutput Vdc from the rectifier DB divided by the impedance element Z2 isapplied as a DC voltage to the series circuit of the impedance elementsZ3 and Z4. A detected voltage Vk derived from the connection point ofthe impedance elements Z3 and Z4 is a voltage corresponding to acombination of an AC component which is a difference between the lampvoltages VLa1 and VLa2 of the two discharge lamps La1 and La2respectively divided by the impedance elements Z1, Z3 and Z4, and a DCcomponent which is the pulsating output Vdc from the rectifier DBdivided by the impedance elements Z2, Z3, and Z4.

When both of the two discharge lamps La1 and La2 are normal, the lampvoltages VLa1 and VLa2 of the lamps La1 and La2 are sinusoidal of thesame amplitude but in out of phase relation to each other by aboutone-half cycle, as shown in FIGS. 5A and 5B, such that the lamp voltagesare cancelled at the connection point of the impedance elements Z3 andZ4, causing the detected voltage Vk to have substantially zero ACcomponent Vk(AC), as shown in FIG. 5C. In this condition, since theconnection point of the Impedance elements Z3 and Z4 sees the DCcomponent Vk(DC) depending upon the dividing ratio of the impendanceelements Z2 to Z4, as seen in FIG. 5, the detected voltage Vk iseventually equal to the DC component Vk(DC).

When, on the other hand, the filament of the discharge lamp La1 becomesdepleted (emitter depletion condition), for example, the filamentradiates only a reduced amount of thermion, whereby the lamp voltageVLa1 of the discharge lamp La1 be comes asymmetric with respect to thezero voltage with a larger amplitude than in the normal condition. Withthis result, no cancellation of the voltages is made at the connectionpoint of the impedance elements Z3 and Z4, whereby an oscillationvoltage appears as the AC component Vk(AC) of the detected voltage Vk,as shown in FIG. 6C. It is noted that the DC component Vk(DC) is keptunvaried, as shown in FIG. 6D. That is, the detected voltage Vk will bethe voltage corresponding to the high frequency AC component Vk(AC)superimposed on the DC component Vk(DC), as shown in FIG. 6E. Therefore,the detected voltage Vk, which is the high frequency AC component Vk(AC)superimposed on the DC component Vk(DC), can be processed such as by apeak detection in order to obtain a purely DC detected voltage Vk′, asshown in FIG. 6F, depending on the lamp voltage VLa1 of the dischargelamp La1 suffering from the depletion of the emitter. Thus obtaineddetected voltage Vk′ is compared with a predetermined threshold Vth suchthat the discharge lamp can be judged to reach the lamp life end whenthe detected voltage Vk′ exceeds the threshold Vth. This judgment ismade at an abnormality detection circuit (not shown) which transmits anabnormality detection signal to a control circuit (not shown) when theabnormality (the lamp life end due to the emitter depletion condition)is detected. In response to the abnormality signal, the control circuitresponds to control the switching elements Q1 and Q2 in such a manner asto intermittently oscillate the inverter circuit for making the circuitprotection.

The present embodiment is contemplated to insert the impedance elementsZ1, Z1 respectively between the one filament of the discharge lamp La1and the node having no high frequency amplitude (the ground) and betweenthe one filament of the discharge lamp La2 and the node, and to detectthe AC component difference between the lamp voltages VLa1 and VLa2 ofthe discharge lamps La1 and La2 in the respective closed loops eachincluding the impedance element Z1 and each of the discharge lamps La1and La2 in order to judge whether or not there is the abnormality due tothe depletion of the emitter. Therefore, it is made possible to detectthe occurrence of the abnormality reliably irrespective of the fact thatthe discharge lamps La1 and La2 give the lamp voltages VLa1 and VLa2 ofvarying amplitudes depending on the temperature, i.e., irrespective ofthe low and high temperature environments. Also, since there is no needto include a DC blocking capacitor on the side of the secondary windingN2 of the leakage transformer LT1, no DC component is applied to thedischarge lamps La1 and La2 so as to prevent the cataphoresisphenomenon. Further, since the present embodiment is configured suchthat the pulsating output Vdc of the rectifier DB has an effect on thedetected voltage Vk′, it is possible to reliably detect the occurrenceof the abnormality even with the use of the inverter circuit of whichoutput varies with the varying AC source voltage, that is, increaseswith the raised AC source voltage and decreases with the lowered ACsource voltage.

(Second Embodiment)

FIG. 7 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment, while FIG. 8 shows acircuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

The present embodiment is contemplated to insert series combinations ofimpedance elements Z1 and Z5 respectively between the filament (a) ofthe discharge lamp La1 and the ground, and between the filament (d) ofthe discharge lamp La2, and also insert a impedance element Z3 alonebetween the filament (c) of the discharge lamp La2 and the ground. Thelike abnormality detection circuit (not shown) is included to judge thepresence of the abnormality with regard to the one discharge lamp La1based upon the detected voltage Vk1 derived from the connection point ofthe impedance elements Z1 and Z5, and to judge the presence of theabnormality with regard to the other discharge lamp La2 based upon thedetected voltage Vk2 derived from the connection point of the impedanceelements Z1 and Z6. When the abnormality is judged fro anyone of thedischarge lamps La1 and La2, the like control circuit (now shown)operates to give the protective action such as by making theintermittent oscillation.

In the present embodiment, the detected voltage Vk1 reflecting the lampvoltage VLa1 of the discharge lamp La1 is used to detect the abnormality(depletion of the emitter), and the detected voltage Vk2 reflecting thelamp voltage VLa2 of the discharge lamp La2 is used to detect theabnormality (depletion of the emitter). Also in the present embodiment,it is equally possible to reliably judge the abnormality irrespective ofthe varying amplitudes of the lamp voltage VLa1 and VLa2 from low tohigh temperature environments as is made in the first embodiment. Also,since the detected voltages Vk1 and Vk2 are made reflective of the DCcomponent of the pulsating output Vdc from the rectifier DB as is madein the first embodiment, it is possible to reliably detect theoccurrence of the abnormality even with the use of the inverter circuitof which output varies with the varying AC source voltage, that is,increases with the raised AC source voltage and decreases with thelowered AC source voltage.

(Third Embodiment)

FIG. 9 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment, while FIG. 10 shows acircuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

The present embodiment is characterized to insert a series combinationof impedance elements Z1 and Z5 between the filament (a) of thedischarge lamp La1 and the ground in order to obtain a detected voltageVk1 derived from the connection point between the impedance elements Z1and Z5, and to obtain a detected voltage Vk2 derived from the connectionpoint between impedance elements Z3 and Z4 such that the likeabnormality detection circuit (not shown) can judge the occurrence ofthe abnormality for the discharge lamps La1 and La2 based upon thedetected voltages Vk1 and Vk2. When the abnormality is judged to occurin the discharge lamps La1 and La2, the like control circuit (not shown)operates to give the protective action such as by making theintermittent oscillation. In the first embodiment, either when thereoccurs the depletion of the emitter in the filament (a) of the dischargelamp La1 connected to the secondary winding N2 and also in the filament(c) of the discharge lamp La2 connected to the auxiliary winding, orwhen there occurs the depletion of the emitter in the filament (b) ofthe discharge lamp La1 connected to the auxiliary winding N3 and also inthe filament (d) of the discharge lamp La2 connected to the secondarywinding N2, the detected voltage Vk has only a small AC component Vk(DC)which makes it difficult to judge the presence of the abnormality.

However, in the present embodiment, the detected voltage Vk2 derivedfrom the connection point of the impedance elements Z3 and Z4 is reliedupon to judge whether anyone of the discharge lamps La1 and La2 reachesthe lamp life end due to the depletion of the emitter, while thedetected voltage Vk1, which is derived from the connection point of theimpedance elements Z1 and Z5 as corresponding to the lamp voltage VLa1of the discharge lamp La1, is relied upon to judge whether both of thedischarge lamps La1 and La2 reach the lamp life end due to the depletionof the emitter. That is, the lamp life end can be judged even in acondition which satisfies both of the events, one in which the depletionof the emitter occurs in the filament (a) of the discharge lamp La1connected to the secondary winding N2 or in the filament (c) of thedischarge lamp La2 connected to the auxiliary winding, and the other inwhich the depletion of the emitter occurs in the filament (b) of thedischarge lamp La1 connected to the auxiliary winding N3, or in thefilament (d) of the discharge lamp La2 connected to the secondarywinding N2.

(Fourth Embodiment)

FIG. 11 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment, while FIG. 12 shows acircuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

The present embodiment, which combines the features of the firstembodiment and the second embodiment, is characterized to insert aseries circuit of impedance elements Z1 and Z5 between the filament (a)of the discharge lamp La1 and the ground, and another series circuit ofimpedance elements Z1 and Z6 between the filament (d) of the dischargelamp La2 and the ground, and to utilize the like abnormality detectioncircuit (not shown) which judges the abnormality in either or both ofthe discharge lamps La1 and La2 based upon a detected voltage Vk1derived from the point to the impedance elements Z2 and Z5 ascorresponding the lamp voltage VLa1 of the discharge lamp La1, upon adetected voltage Vk2 derived from the connection point of the impedanceelements Z3 and Z4, and upon a detected voltage Vk3 derived from theconnection point of the impedance elements Z1 and Z6 as corresponding tothe lamp voltage VLa2 of the discharge lamp La2.

With the present embodiment, it is possible to judge the abnormality inall events including the depletion of the emitter in anyone of thedischarge lamps but also in both of the discharge lamps La1 and La2.

(Fifth Embodiment)

FIG. 13 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment, while FIG. 14 shows acircuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

The present embodiment utilizes capacitors C101 and C102 as theindividual impedance elements Z1 and Z1, and a resistor 109 connectedbetween the capacitors C101, C102 and the ground. The resistor 109limits a high frequency signal flowing through capacitors C101 and C012to the ground in the normal operating condition of the discharge lampsLa1 and La2, reducing circuit noises. An inductor may be utilizedinstead of resistor 109.

Also, a peak detection circuit P is provided to convert the detectedvoltage Vk at the connection point of the impedance elements Z3 and Z4respectively in the form of resistors R101 and R102 into a detected DCvoltage Vk′. The peak detection circuit P includes a series circuit of aDC blocking capacitor C401 and a diode D402 connected to the pointbetween the resistors R101 and R102, a diode D401 inserted between theground and the connection point of capacitor C401 and diode D401, and asmoothing capacitor C402 connected between the cathode of diode D402 andthe ground. Thus, the capacitor C401 DC cuts out the DC component Vk(DC)of the detected voltage Vk so as to charge C402 with energycorresponding to the peak value of the AC component Vk(AC) of thedetected voltage Vk, thereby effectively obtaining the detected voltageVk′ having only the DC component corresponding to the difference in thelamp voltages VLa1 and VLa2 of the discharge lamps La1 and La2. As isexplained with reference to the first embodiment, the detected voltageVk′ is compared with the predetermined threshold Vth such that thedischarge lamps La1 and La2 can be judged to reach the lamp life endwhen the detected voltage exceeds the threshold Vth.

(Sixth Embodiment)

FIG. 15 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment. A basic configurationof the present embodiment is identical to the fifth embodiment andtherefore no duplicate explanation is made herein. The like parts aredesignated by the like reference numerals. Here, only the characterizingfeatures of the present embodiment will be now explained.

The present embodiment is characterized in that capacitors C501 and C502are utilized respectively as impedance elements Z1 and Z1, and also actas the resonant inducing capacitor C2 to dispense with capacitor C2. Thecircuit operation such as for detecting the depletion of the emitter isidentical to the fifth embodiment and therefore its explanation is notmade herein.

Thus, the present embodiment has an advantage of reducing the number ofthe components as the capacitors C501 and C502 are utilized as theimpedance elements Z1 and also as the resonant inducing capacitor C2.

(Seventh Embodiment)

FIG. 16 is a partially omitted schematic circuit diagram showing thepresent embodiment which is basically similar to the second prior art ofFIG. 2. Therefore, like configuration common to the second prior art isnot shown and no duplicate explanation is made herein. Like parts aredesignated by like reference numerals. Here, only the characterizingfeatures of the present embodiment will be now explained.

As shown in FIG. 16, a resistor R1 is inserted between the high-sideoutput terminal of the rectifier DB and a connection point of capacitorC6 connected to the auxiliary winding N3 of the leakage transformer LT1and one filament (b) of the discharge lamp La1. A parallel circuit of acapacitor C8 and a resistor R5 is connected in series with resistors R3and R4 between the ground and the connection point of the auxiliarywinding N3 and the filament (c) of the discharge lamp La2. Further, theswitching element Q2 has its gate connected through a triggering elementTD such as Diac to the connection point of resistor R4 and capacitor C8,while a series circuit of a diode D11 and a resistor R10 is insertedbetween the drain of the switching element Q2 and the connection pointof resistor R4 and capacitor C8. A series combination of the triggeringelement TD, diode D11 and resistor R10 constitutes a starting circuitfor turning on the switching element Q2 when the AC source voltage AC isapplied so as to start the inverter. The like peak detection circuit Pas explained with reference to the fifth embodiment is connected to thepoint between resistors R3 and R4 to derive a detected voltage Vk fromthe connection point.

When the AC source voltage is applied, the rectifier DB charge capacitorC8 through resistor R1, filament (b) of discharge lamp La1, filament (c)of discharge lamp La2, and resistors R3 and R4. When voltage acrosscapacitor C8 increases to the break voltage of the triggering elementTD, the triggering element responds to break-down for supplying thecharge of capacitor C8 to the gate of switching element Q2, therebyturning on switching element Q2 and therefore starting the invertercircuit. When the switching element Q2 is turned on, capacitor C8 isdischarged through diode D11, resistor R10 and switching element Q2 sothat the inverter circuit continues to oscillate. If the filament (b) ofthe discharge lamp La1 or the filament (c) of the discharge lamp La2 isbroken, or if anyone of the discharge lamps La1 and La2 is disconnected(in no-load condition) at the time of emerging the device, no chargingpath is established for capacitor C8. Consequently, in view of thatcapacitor C8 is shunt by resistor R5, the triggering element TD wouldnot break-down and therefore the inverter circuit would not start. Thus,the inverter circuit is prevented from starting at the no-load conditionfor protection of the circuit at the no-load condition.

As explained in the above, since the starting circuit for the invertercircuit of the present embodiment includes the no-load detecting andcircuit protective function of dealing with the broken filaments and thedisconnection of the discharge lamps La1 and La2, in addition to theabnormality detection and protection function of dealing with thedepletion of the emitter, the circuit components can be reducedsignificantly in number.

(Eighth Embodiment)

FIG. 17 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment, while FIG. 18 shows acircuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the second priorart of FIG. 2 as well as to the seventh embodiment. Therefore noduplicate explanation is made herein but the like parts are designatedby the like reference numerals. Here, only the characterizing featuresof the present embodiment will be now explained.

The present embodiment is configure to insert impedance elements Z1 andZ1 respectively between the filament (a) of the discharge lamp La1 andthe ground and between the filament (c) of the discharge lamp La2, andinsert a series combination of impedance elements Z3 and Z4 between theground and the connection point of the auxiliary winding N3 and thefilament (c) of the discharge lamp La2. Also, the like peak detectioncircuit P as explained with reference to the fifth embodiment isconnected to a connection point of the impedance elements Z3 and Z4 sothat the detected voltage Vk derived from the connection point of theimpedance elements Z3 and Z4 is converted into a DC detected voltageVk′,

The control circuit CNT compares the detected voltage Vk′ from the peakdetection circuit P with a predetermined threshold Vth so as to judgethat the discharge lamp La1 or La2 reaches the lamp life end when thethreshold Vth is exceeded, and makes the protective action ofintermittently oscillating the inverter circuit.

Thus, in the like manner as in the first embodiment, the presentembodiment includes the impedance elements Z1 and Z2 which are insertedrespectively between the one filament of the one discharge lamp La1 andthe node having no high frequency amplitude (the ground), and betweenthe one filament of the other discharge lamp La2 and the ground, inorder to detect a difference in the AC component of the lamp voltagesVLa1 and VLa2 of the discharge lamps La1 and La2 within the closed loopseach including the impedance element Z1 and each of the discharge lampsLa1 and La2, for the purpose of judging the abnormality due to thedepletion of the emitter. Accordingly, it can be made to reliably judgethe abnormality irrespective of the varying amplitudes of the lampvoltage VLa1 and VLa2 from low to high temperature environments. Also,since there is no need to include a DC blocking capacitor on the side ofthe secondary winding N2 of the leakage transformer LT1, no DC componentis applied to the discharge lamps La1 and La2 so as to prevent thecataphoresis phenomenon.

(Ninth Embodiment)

FIG. 19 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment. The present embodimentincludes a rectifier DB in the form of a diode bridge responsible for afull-wave rectification of an AC source voltage AC to provide apulsating output that is smoothed by a smoothing capacitor C1 to give avoltage source for an inverter circuit. The inverter circuit is of aso-called half-bridge configuration and includes a series combination ofswitching elements Q1 and Q2 respectively in the form of bipolartransistors connected across the smoothing capacitor C1, diodes D1 andD2 each connected in anti-parallel relation across each of the switchingelements Q1 and Q2, and a series circuit of capacitors C3 and C4connected across the smoothing capacitor C1. Connected to a pointbetween capacitors C3 and C4 is a series circuit of a primary winding N1of a leakage transformer LT1 and a primary winding of a drivingtransformer T1 which is provided for driving the switching elements Q1and Q2. The leakage transformer LT1 has a secondary winding N2 connectedto filaments (a) and (d) of the discharge lamps La1 and La2, and anauxiliary winding N3 connected to filaments (b) and (c) of the dischargelamps La1 and La2. A resonant inducing capacitor C5 is connected tofilaments (a) and (d) of the discharge lamps La1 and La2 on thenon-energized side thereof. Instead of using the combination of thebipolar transistors and the diodes D1 and D2, the switching elements Q1and Q2 may be realized by field effect transistors having parasiticdiodes.

The switching elements Q1 and Q2 are activated by the drivingtransformer T1 to turn on and off alternately with the switchingelements Q1 and Q2 being responsible for flowing currents in oppositedirections through the leakage transformer LT1 to the discharge lampsLa1 and La2 respectively from capacitors C3 and C4, thereby applying ahigh frequency voltage developed across capacitor C5 resulting from aseries resonant circuit of a leakage inductance and capacitor C5 forstarting and operating the lamps.

Also in the present embodiment, a capacitor C8 is inserted as theimpedance element between the filament (a) of the discharge lamp La1 anda node (the ground) having no high frequency amplitude, while acapacitor C9 is inserted as the impedance element between the filament(b) of the discharge lamp La2 and the node (the high-side outputterminal of rectifier DB). Further, an emitter depletion detection andprotection circuit 10 is connected between a connection of a baseresistor R2 of the switching element Q2 with the secondary winding ofthe driving transformer T1 and the auxiliary winding N3 in order todetect the depletion of emitter in anyone of the filaments (a) to (d) ofthe discharge lamps La1 and La2 for protection of the circuit.

The emitter depletion detection and protection circuit 10 includes aseries circuit of a DC blocking capacitor C7 and a diode D6 connectedbetween the filament (c) of the discharge lamp La2 and the ground, adiode D5 having an anode connected to a cathode of diode D6 connected tothe capacitor C7, a zener diode ZD1 having a cathode connected to thecathode of diode D5, and a parallel combination of a smoothing capacitorC6 and a discharging resistor R5 connected between the cathode of zenerdiode ZD1 and the ground. A capacitor C10 is connected in parallel witha biasing resistor R4 between the anode of zener diode ZD1 and theground, while a switching element Q3 of PNP-type bipolar transistor isconnected in series with a diode D7 between the base resistor R2 of theswitching element Q2 and the resistor R4. Further, a biasing resistor R3is connected in an emitter-base path of the switching element Q3, whilea switching element Q4 of NPN-type bipolar transistor is connectedbetween resistor R3 and the switching element Q4.

As the capacitor C8 is inserted between the filament (a) of thedischarge lamp La1 and the ground and the capacitor C9 is insertedbetween the filament (d) of the discharge lamp La2 and the high-sideoutput terminal of the rectifier DB, the high frequency currentsrespectively flowing through the discharge lamps La1 and La2 becomesasymmetrical with each other if anyone of the filaments (a) to (d) ofthe discharge lamps La1 and La2 sees the depletion of the emitter. Theresulting asymmetrical high frequency currents are responsible forcharging the capacitor C7 and the capacitor C6 through diode D5. Whenthe voltage across capacitor C6 exceeds the zener voltage of zener diodeZD1, capacitor C6 is discharged to turn on the switching element Q4,which in turn causes the switching element Q3 to turn on, therebyconnecting the secondary winding of the driving transformer T1 fordriving the switching element Q2 to the ground through diode D7. Withthis result, the switching element Q2 becomes not capable of turning onto stop the inverter circuit. Thus, the emitter depletion detection andprotection circuit 10 can detect the depletion of the emitter of thedischarge lamps La1 and La2, and stops the inverter circuit forprotection of the circuit upon detection of the depletion of theemitter.

In the present embodiment, the impedance elements C8 and C9 are insertedrespectively between the filaments of the discharge lamps La1, La2 andthe nodes having no high frequency amplitude (the ground or thehigh-side output terminal of rectifier DB) in order to detect theasymmetric high frequency currents at the connection between thedischarge lamps La1 and La2 for judging whether there occurs thedepletion of emitter. Therefore, it is possible to reliably judge theoccurrence of the depletion of the emitter irrespective of whether it isoperating in the low or high temperature environment. Further, sincethere is no need to connect a DC blocking capacitor to the secondarywinding N2 of the leakage transformer LT1, the discharge lamps La1 andLa2 can be free from the DC component so as to be prevented from causingthe cataphoresis phenomenon.

It may be equally possible to insert capacitors C8 and C9 respectivelybetween the filament (a) of the discharge lamp La1 and the high-sideoutput terminal of the rectifier DB, and between the filament (d) of thedischarge lamp La2 and the high-side output terminal, as shown in FIG.20; to insert capacitors C8 and C9 respectively between the filament (a)of the discharge lamp La1 and the ground, and between the filament (d)of the discharge lamp La2 and the ground, as shown in FIG. 21; to insertresistors Ra and Rd instead of capacitors C8 and C8 between therespective filaments (a) and (d) of the discharge lamps La1 and La2 andthe respective one of the high-side output terminal of rectifier DB andthe ground, as shown in FIG. 22; or even to use a series combination ofresistor and capacitor as the impedance element. In any case, the highfrequency currents flowing through the discharge lamps La1 and La2becomes asymmetrical with each other when there occurs the depletion ofthe emitter in anyone of the filaments (a) to (d) of the discharge lampsLa1 and La2 so that the emitter depletion detection and protectioncircuit 10 can responds to detect the asymmetrical high frequencycurrents for judging whether or not there occurs the depletion of theemitter.

(Tenth Embodiment)

FIG. 23 shows a schematic circuit diagram of the discharge lamp drivingdevice in accordance with the present embodiment which is basicallysimilar to the second prior art of FIG. 2. Therefore, like configurationcommon to the second prior art is not shown and no duplicate explanationis made herein. Like parts are designated by like reference numerals.Here, only the characterizing features of the present embodiment will benow explained.

In the present embodiment, capacitor C8 is inserted as the impedanceelement between the filament (a) of the discharge lamp La1 and the node(the high-side output terminal of rectifier DB) having no high frequencyamplitude, while capacitor C9 is inserted as the impedance elementbetween the filament (d) of the discharge lamp La2 and the node(ground). Also, connected between the gate of the switching element Q2and the auxiliary winding N3 is the like emitter depletion detection andprotection circuit 10 which detects the depletion of the emitter inanyone of the filaments (a) to (d) of the discharge lamps La1 and La2for protection of the circuit. The emitter depletion detection andprotection circuit is identical in configuration and operation to thatof the ninth embodiment, and therefore no duplication explanation ismade.

Similar to the ninth embodiment, the present embodiment is configured toinsert capacitor C8 between the filament (a) of the discharge lamp La1and the high-side output terminal of rectifier DB, to insert capacitorC9 between the filament (d) of the discharge lamp La2 and the ground,and to provide the emitter depletion detection and protection circuit 10which detects the asymmetric high frequency currents at the connectionbetween the discharge lamps La1 and La2 for judging whether there occursthe depletion of the emitter. Therefore, it is possible to reliablyjudge the occurrence of the depletion of the emitter irrespective ofwhether it is operating in the low or high temperature environment.Further, since there is no need to connect a DC blocking capacitor tothe secondary winding N2 of the leakage transformer LT1, the dischargelamps La1 and La2 can be free from the DC component so as to beprevented from causing the cataphoresis phenomenon.

The inverter circuit may be of different circuit configurationsincluding, for example, one in which the resonant load circuit isconnected between the connection point of the switching elements Q1 andQ2 and the low-side output terminal of the rectifier DB, and one inwhich a valley-filling power source composed of a voltage doubler isutilized instead of the valley-filling power source composed of thestep-down chopper circuit. The concept of the present invention can beapplied

It is noted that the concept of the present invention can be applied tovarious circuit configurations of the inverter circuit. For example, theinverter circuit may be of different configurations including one inwhich the resonant load circuit is connected between the connectionpoint of the switching elements Q1 and Q2 and the low-side outputterminal of the rectifier DB, and one in which a valley-filling powersource composed of a voltage doubler is utilized instead of thevalley-filling power source composed of the step-down chopper circuit.

What is claimed is:
 1. A discharge lamp driving device comprising: arectifier which rectifies an AC source voltage; a smoothing capacitorwhich smoothes out a pulsating output of the rectifier; an invertercircuit having at least one switching element for conversion of thesmoothed DC output made through the smoothing capacitor into a highfrequency output; a load circuit including a resonance circuit and adischarge lamp and being supplied with the high frequency output fromthe inverter circuit; an output transformer having a primary connectedto an output end of the inverter circuit and having a secondaryconnected to one filament end of the discharge lamp; an impedanceelement inserted respectively between the other filament end of thedischarge lamp and a node having no high frequency amplitude; and anabnormality detection and protection means which detects an amplitude ofthe high frequency output flowing through the discharge lamp and theimpedance element in order to make a circuit protection when thedetected amplitude exceeds a predetermined threshold.
 2. The dischargelamp driving device as set forth in claim 1, wherein a plurality of saiddischarge lamps are connected in series across the secondary of saidoutput transformer, the impedance element being inserted between theother filament of at least one of said discharge lamps and the positiveside input terminal of the inverter circuit, and another impedanceelement being inserted between the other filament end of at leastanother said discharge lamp and the grounded input terminal or outputterminal of the inverter circuit.
 3. The discharge lamp driving deviceas set forth in claim 1, wherein a plurality of said discharge lamps areconnected in series across the secondary of said output transformer,said abnormality detection and protection means making the circuitprotection when the amplitude of the high frequency output flowing atleast one of said discharge lamps and the impedance element exceeds apredetermined threshold.
 4. The discharge lamp driving device as setforth in claim 1, wherein a plurality of said discharge lamps areconnected in series across the secondary of said output transformer,said abnormality detection and protection means detecting the amplitudeof the voltage at the connection between the filaments of the individualdischarge lamps, detecting the amplitude of the high frequency outputflowing through at least one of said discharge lamps and the impedanceelement, and making the circuit protection when at least one of saidamplitudes exceeds a predetermined threshold.
 5. The discharge lampdriving device as set forth in claim 1, wherein a plurality of saiddischarge lamps are connected in series across the secondary of saidoutput transformer, said abnormality detection and protection meansdetecting the amplitude of the voltage at the connection between thefilaments of the individual discharge lamps, and making the circuitprotection when at the amplitude at said connection or an amplitude of ahigh frequency output flowing through at least one of the high-voltageand low-voltage side discharge lamps and through the impedance elementexceeds a predetermined threshold.
 6. The discharge lamp driving deviceas set forth in claim 1, wherein said impedance element is a resistor.7. The discharge lamp driving device as set forth in claim 1, whereinsaid impedance element is a capacitor.
 8. The discharge lamp drivingdevice as set forth in claim 1, wherein said impedance element is aseries combination of a resistor and a capacitor.
 9. The discharge lampdriving device as set forth in claim 1, wherein said inverter circuit isof a self-excited type, and a starting circuit for starting the invertercircuit shares at least a portion thereof with said abnormalitydetection and protection means.
 10. The discharge lamp driving device asset forth in claim 1, wherein said impedance element is shared with aresonant circuit included in the load circuit.
 11. The discharge lampdriving device as set forth in claim 1, wherein a plurality of saiddischarge lamps are connected in series across the secondary of saidoutput transformer.
 12. The discharge lamp driving device as set forthin claim 11, wherein the impedance elements inserted between thefilaments of the respective discharge lamps and the node having no highfrequency amplitude have substantially the same impedance value.
 13. Thedischarge lamp driving device as set forth in claim 1, wherein saidimpedance element is inserted between said other filament end of thedischarge lamp and a positive side input terminal of the invertercircuit.
 14. The discharge lamp driving device as set forth in claim 13,wherein a plurality of said discharge lamps are connected in seriesacross the secondary of said output transformer.
 15. The discharge lampdriving device as set forth in claim 14, wherein the impedance elementsinserted between the filaments of the respective discharge lamps and thenode having no high frequency amplitude have substantially the sameimpedance value.
 16. The discharge lamp driving device as set forth inclaim 1, wherein said impedance element is inserted between said otherfilament end of the discharge lamp and a grounded input terminal oroutput terminal of the inverter circuit.
 17. The discharge lamp drivingdevice as set forth in claim 16, wherein a plurality of said dischargelamps are connected in series across the secondary of said outputtransformer.
 18. The discharge lamp driving device as set forth in claim17, wherein the impedance elements inserted between the filaments of therespective discharge lamps and the node having no high frequencyamplitude have substantially the same impedance value.